Three-pole or four-pole low-voltage power switches are sometimes fitted with a device for ground-fault detection. To detect a ground fault, it is necessary to form the vectorial sum of the currents in the three/four conductors of the monitored electrical power network. It is also possible to connect an external summation current transformer, which summates the primary currents directly. Depending on the requirements of the switching installation in which the power switch is used, a detected ground fault is either merely signaled or causes the switch to trip after a preset delay.
If the power switch works using Rogowski coils as current sensors, as disclosed in German patent DE 100 54 496 A1 for example, the currents in the Rogowski coils are far too small for a summation evaluation according to the type of detection in the neutral conductor described in the introduction. In order to still be able to detect ground faults, the practice until now was to generate by calculation the summation current from the individual current signals, after their analog-to-digital conversion in the microprocessor, and to provide it as a digital value.
The sampling in the processor results in errors, because the processor detects the individual analog-to-digital converter values with a time offset from one phase to the next. Thus, the signal is measured with a time difference of 90 μs from phase L1 to phase N. Although the summation current can be calculated by interpolation and backward projection to the first of three/four sample values in each case, a significant systematic error still remains, in particular when there are dynamic current variations in the power network.